Sheck Exley
Wrote the rules that keep cave divers alive, then died breaking his own depth record
Sheck Exley began cave diving as a teenager in the network of flooded limestone caves and sinkholes that riddle northern Florida, a sport then so young and poorly understood that many of its early practitioners, including friends of his, died working out its dangers by trial and error. Rather than treat those deaths as unavoidable, Exley set out to analyse exactly what had gone wrong in each one and turn the lessons into rules other divers could actually follow.
His writing, particularly Basic Cave Diving: A Blueprint for Survival, became foundational to the sport, formalising practices like the "rule of thirds," which reserves two-thirds of a diver's air supply for the return journey regardless of how the exploration is going, and standardising the use of a backup, or octopus, regulator that has since become basic equipment for divers generally, not only cave specialists.
He combined this caution with genuine ambition, setting a long series of world depth and distance records in cave and sinkhole diving in Florida and later in Mexico's deep cenotes, pushing the boundaries of what was survivable while trying, wherever possible, to do so within the safety margins he had spent years defining for others.
In April 1994 he attempted a depth record in the Mexican sinkhole Zacatón, one of the deepest known underwater caves in the world. He failed to return, having reached an estimated depth of around 276 metres; his diving partner Jim Bowden, diving separately, went deeper still and survived, spending over nine hours decompressing on the way back up. Exley's death, at the frontier of a sport he had spent his life trying to make safer, was felt as a genuine loss across the cave diving community.